LIGO is designed to detect gravitational waves by measuring their effect using two L-shaped detectors about 3,200 kilometres apart. Passing gravitational waves are expected to make light take slightly longer to travel in one direction than the other. To us, that make it looks like a decrease in the length of one arm of the L and an increase in the other. The minuscule change is measured using lasers and mirrors. A real gravitational wave should be detected by both detectors.